Effect of protein-tannin ratio and tannin concentration on the bovine serum albumin (BSA)-based precipitation method for red wine tannin concentration

Aims: The main objective of the study was to ascertain whether an existing protein precipitation assay could be simply modified to determine tannin at low concentration in wines. This was achieved by mixing a greater volume of wine to a smaller, but more concentrated, volume of bovine serum albumin (BSA) to maintain the same wine-to-BSA ratio (although both the final pH and ethanol concentration varied). In addition, dilution series of each of these mixtures were prepared to investigate the effect of wine-to-BSA ratio on tannin precipitation. Methods and results: Seven New Zealand red wines were assayed according to the BSA method using a range of protein (BSA) and wine concentrations achieved by varying wine dilutions and the volume of the model wine solution. Maximum precipitation was observed at lower wine/protein ratios in diluted wines and tannin precipitation increased as protein concentration increased. It was observed that the estimation of tannin concentration in red wine is a product of tannin/protein ratio and BSA concentration. Consequently, the methylcellulose precipitation (MCP) assay was performed to independently determine tannin concentration in red wines. Results indicate that tannin/protein ratio, BSA concentration and possibly tannin composition affect BSA-tannin precipitation. Conclusion: For the BSA assay there appears to be a region of low tannin/protein ratio within which lower wine tannin concentrations can be determined. Overall it is suggested that tannin precipitation is linearly related to tannin concentration. Significance and impact of the study: Results showed the limits of the BSA method for low tannin wines and the difficulty in using the method for wines with unknown tannin concentrations